> Date: Tue, 9 Sep 2008 19:10:45 -0700 (PDT)
> From: Steve Noskowicz
> Subject: Re: [aprssig] (Slightly OT) What is solar panel
> power output?
>> Bob,
> Correct me if I am wrong, but aren't good solar
> cells closer to a (Pretty) constant voltage *UNTIL* you get
> close to the current limit, then they supply *near* constant
> current down to zero volts?
> There must be some "softness" to this, In other
> words, *some* voltage decrease as the load increases,
> probably some IR losses.
> Then as load increases further, you have a knee where the
> Voltage starts dropping faster and faster as the current
> increases slightly to Isc. [or is there some foldback?]
> However, I believe this is what you are saying.
>> --
> 73, Steve, K9DCI
Yep, that's just about what he said. There's a gentle taper to the terminal voltage starting from the open circuit voltage as current is drawn off the cell. This begins to accelerate as you get closer to the maximum power point and then it drops nearly instantly to zero volts as you approach the short circuit current of the cell. Pmax has to be determined experimentally for each cell as it's difficult to predict exactly where that point lies. The fill factor is the figure of merit for determining the shape of the knee and the tapers you see. The cutoff between a good and a bad cell is a fill factor of around 0.7 (maximum is 1.0). The conversion efficiency will be the determination of what Pmax actually is given one-sun conditions.
Of course one of the biggest factors contributing to a loss of power production in a solar cell is heat. Heat increases resistances in the cell and lowers efficiency while eventually damaging the cell. Keep your panels as cool as possible for maximum performance and life.